Evolution of heteromorphic sex chromosomes in the order Aulopiformes

The fish order Aulopiformes contains both synchronously hermaphroditic and gonochoristic species. From the cytogenetic viewpoint, few reports show tha t gonochoristic Aulopiformes have heteromorphic sex chromosomes. Because fi sh in this order give us a unique opportunity to elucidate the evolution of sex chromosomes, it is important to examine a phylogenetic relationship in Aulopiformes by both molecular evolutionary and cytogenetic methods. Thus, we conducted molecular phylogenetic and cytogenetic studies of six Aulopif orm species. Our results suggested that hermaphroditic species were evoluti onarily derived from gonochoristic species. It follows that the hermaphrodi tic species might have lost the heteromorphic sex chromosomes during evolut ion. Here, we suggest a possibility that heteromorphic sex chromosomes can disappear from the genome. even if they have appealed once in evolution. Ta king into account Ohno's hypothesis that heteromorphic sex chromosomes migh t have emerged from autosomes. we propose the hypothesis that heteromorphic sex chromosomes may have undergone repeated events of appearance and disap pearance during the course of fish evolution. (C) 2000 Elsevier Science B.V . All rights reserved.